Libvirt driver iothread property for virtio-scsi disks

[Nir Soffer]

The docs[1] say:

- The optional iothread attribute assigns the disk to an IOThread as defined by
  the range for the domain iothreads value. Multiple disks may be assigned to
  the same IOThread and are numbered from 1 to the domain iothreads value.
  Available for a disk device target configured to use "virtio" bus and "pci"
  or "ccw" address types. Since 1.2.8 (QEMU 2.1)

Does it mean that virtio-scsi disks do not use iothreads?

I'm experiencing a horrible performance using nested vms (up to 2 levels of
nesting) when accessing NFS storage running on one of the VMs. The NFS
server is using scsi disk.

My theory is:
- Writing to NFS server is very slow (too much nesting, slow disk)
- Not using iothreads (because we don't use virtio?)
- Guest CPU is blocked by slow I/O

Does this make sense?

[1] https://libvirt.org/formatdomain.html#hard-drives-floppy-disks-cdroms

Nir